154 SYDNEY H. VINES. 



of the development of the lichen-thallus from the germination 

 of the spore to the formation of apothecia. 



These hymenial gonidia are, as their name denotes, gonidia 

 which occur in the hymenial layer of the apothecium in certain 

 Lichens. Nylander seems to have been the first to observe them, 

 and the subsequent investigations of Fuisting and of Winter, 

 have shewn that they originate from the ordinary gonidia of the 

 thallus. They differ from the gonidia of the thallus in that they 

 are smaller and are of a less vivid colour. The primary object 

 of StahPs researches was to account for this difference in appear- 

 between these two kinds of gonidia, and to discover the real 

 significance of the presence of gonidia in the hymenium. 



The Lichen which he more especially studied was Ihidocarpon 

 pnsilbim (Hedwig). In its thalJus the gonidia, which are of a 

 uniform green colour, form a layer lying between the medullary 

 and cortical parts and extending here and there for a greater or 

 less distance into the cortex. By their mode of division they 

 indicate their connection with the algal genus Pleurococcus. The 

 perithecia present the structure which is usually found in pyreno- 

 carpous Lichens. In the spaces between the asci (Fig. 1), andmore 

 or less fining up its whole cavity, is a gelatinous substance which is 

 produced by the swelling up of the membranes of emptied asci, 

 in which lie the numerous pale-green hymenial gonidia. They 

 differ remarkably in size from the gonidia of the thallus, having 

 a diameter of only 0*002 — 0004 m.m., whereas the diameter of 

 the latter is from 0008 — 0"012 m.m. Each ascus usually con- 

 tains two spores of unequal size, the upper one being rather 

 larger than the lower. When ripe, they are brown, multicellular, 

 pseudo-parenchymatous structures of considerable size. When 

 moistened, the asci and the gelatinous substance surrounding 

 them absorb a quantity of water and swell-up. In consequence 

 of the tension thus produced within the perithecium, the ripe 

 asci burst and expel their spores with such force that they are 

 projected to a distance of several centimeters. 



An examination of the spores which have been thus extruded 

 shews that they are surrounded by a number of the pale-green 

 hymenial gonidia. (Fig. 2). If a spore be kept moist, it germi- 

 nates at once after its extrusion, and if it be placed upon a glass 

 slide the wiiole process can be easily observed. A certain number 

 of the hyphae which spring from the segments of the spore 

 invest the hymenial gonidia by which the spores is surrounded 

 (Fig. 3), and in a few days the invested gonidia shew that they 

 have undergone considerable change. The previously scanty 

 chlorophyll which gave them a pale-green colour has increased in 

 quantity, so that the enlarged gonidia are uniformly dark-green. 

 A comparison of the free and of the enclosed goindia shewn in 



